Surface-hardened titanium and titanium alloys and method of processing same

ABSTRACT

Surface-hardened pure titanium or titanium-base alloys having a coating of a substitution type metal, which metal is characterized by a larger diffusion constant than titanium, is more readily nitrided than titanium, has a higher density than the base metal, is rich in stable beta phase, and which has a Vickers hardness of above 400. This hardened titanium or titanium-base alloy is produced by a method of hardening the surface of pure titanium or a titanium-base alloy which includes the steps of coating a substitution type metal of the abovementioned type and heating the coated metal in a nitrogencontaining atmosphere.

United States Patent [1 1 Tokuda et a1.

[ SURFACE-HARDENED TITANIUM AND TITANIUM ALLOYS AND METHOD OF PROCESSINGSAME [75] Inventors: Shoichi Tokuda; Hiromichi Kawahara, both of Kobe,Japan [73] Assignees Kobe Steel, Ltd., Kobe City, Japan [22] Filed: Mar.22, 1971 [21] Appl. No.: 126,634

[52] US. Cl 148/203, 29/198, 148/31.5, 148/325, 148/133 [51] Int. Cl.C23c 11/14, C22c 15/00 [58] Field of Search 148/13.1, 15.5, 16.6,148/203, 31.5, 32.5, 133; 75/175.5; 29/198; 204/37, 38

[56] References Cited UNITED STATES PATENTS 3,471,342 10/1969 Wood148/315 X 1,929,252 10/1933 Morris 148/166 2,864,731 12/1958 Gurinsky eta]. 148/15 X 1,944,179 1/1934 Homerberg 148/166 1,936,294 11/1933 Egan148/166 2,858,600 11/1958 Vigor 29/198 [451 Oct. 16, 1973 7/1953 Herres148/133 X 8/1960 Steinberg 204/37 OTHER PUBLICATIONS PrimaryExaminerCharles N. Lovell Attorney-Oblon, Fisher & Spivak [5 7] ABSTRACTSurface-hardened pure titanium or titanium-base alloys having a coatingof a substitution type metal, which metal is characterized by a largerdiffusion constant than titanium, is more readily nitrided thantitanium, has a higher density than the base metal, is rich in stablebeta phase, and which has a Vickers hardness of above 400. This hardenedtitanium or titanium-base alloy is produced by a method of hardening thesurface of pure titanium or a titanium-base alloy which includes thesteps of coating a substitution type metal of the above-mentioned typeand heating the coated metal in a nitrogen-containing atmosphere.

6 Claims, 9 Drawing Figures PAIENTEnncI 16 ms SHEET 10F 3 FIG. 1

mvsmos SHOICHI TOKUDA 0nd HIROMICHI KAWAHARA ATTORNEYS FIG. 2B

PATENTEUUBI 16 I973 SHEET 3 OF 3 has E54 $2 5 205 6 5 mmwzx BOO 100THICKNESS 0F HARDENED LAYER OF TITANIUM (1*) DTSTANCE FROM THE SURFACEOF IRON PLATED TlTANlUM-BASE 28s wmmzom 5;

ALLOY (m FIG. '5

DISTANCE FROM THE SURFACE OF NTCKEL OR COPPER PLATED TITANIUM-BASE ALLOYFIG] SURFACE-HARDENED TITANIUM AND TITANIUM ALLOYS AND METHOD OFPROCESSING SAME BACKGROUND OF THE INVENTION 1. Field of Invention Thisinvention relates to surface-hardened pure titanium or titanium-basealloys and to a method of processing same. More particularly, thisinvention relates to surface-hardened pure titanium or titanium-basealloys having a relatively harder and thicker surface, and to a methodof hardening said surface.

2. Description Of The Prior Art Since titanium and titanium-base alloyshave superior corrosion resistance, relatively low specific weight, andhigh tensile strength, they are used in strengthening members ofaircraft and spacecraft and as materials in the rotary portions of a jetengine. Since they also have proper refractoriness, they have also beenused as materials in various devices and equipment in the chemicalindustry. Thus, titanium and titanium-base alloys are broadly used invarious fields.

However, titanium and titanium-base alloys have, as their chiefdisadvantage, a very seizable property, and accordingly arecharacterized by a low wear resistance, thereby rendering themunsuitable for those applications which require high wear resistance.

In order to overcome these disadvantages of titanium and titanium-basealloys, it is common to surfaceharden the metal or to plate the metalwith a different metal to obtain increased wear resistance.

Commonly used surface-hardening methods include nitriding treatment,oxidation treatment, surfacehardening treatment for depositing titaniumin an inert gas including small amounts of nitrogen and oxygen, andcarbonization treatment. As methods of coating a different metal, thereare chrome or nickel plating treatments and high hardness special alloywelding treatments.

These methods are adapted for practical use for their respectiveutilities, but each of these methods has corresponding disadvantages inaddition to their advantages. For instance, when the surface isnitrided, the thickness of the nitrided layer is usually approximately40 microns, in the case of pure titanium (heated at a temperature of850C. for 100 hours), plus or minus approximately microns, dependingupon the type (heated at a temperature of 850C. for 100 hours). Sincethese heat treatments are conducted at a high temperature for longperiods of time, the treated metal tends to be deformed. If thedeformation is corrected, such as by polishing, the hardened layer willbe disadvantageously removed.

In the oxidation treatment, the oxidized layer, obtained in a moltenglass bath, is even thicker than the nitrided layer, but theinterstitial type elements for hardening of titanium, such as oxygen andnitrogen, abruptly reduce its hardening capacity if the temperaturebecomes 250300C., with the resultant disadvantage that the effectivenessof this surface-hardening treatment is extremely decreased. Even asurfacehardening treatment similar to the aforementioned oxidizingtreatment has disadvantages, in that blow-holes may form duringsurface-hardening, and machining is required to form the final shapeafter surfacehardening.

Though the carbonization treatment increases the hardness of the surfaceby the formation of carbonized titanium, the hardened layerdisadvantageously becomes porous and fragile.

Though chrome and nickel plating methods are effective, it is not alwayseasy to obtain sufficient exfoliation strength in the plated layer, andhydrogen absorption by the plating occurs to cause so-called hydrogenfragility and an extreme reduction of the fatigue strength of the basemetal.

Thus, none of the aforesaid methods improve the wear resistance oftitanium and titanium-base alloys in a completely satisfactory manner.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toeliminate the aforementioned disadvantages of the conventionallysurface-hardened titanium, and titanium-base alloys and methods ofprocessing same by providing novel and improved surface-hardened puretitanium and titanium-base alloys and a method of producing the same.

Another object of the present invention is to provide surface-hardenedpure titanium and titanium-base al- Ioys which have hardened layers ofboth sufficient surface hardness and thickness.

It is another object of the present invention to provide a method ofhardening the surface of pure titanium and titanium-base alloys.

Another object of the present invention is to provide a method ofhardening the surface of pure titanium and titanium-base alloys whichcomprises the steps of coating a substitutional metal thereon andheating the coated metal in an atmosphere including nitrogen.

According to a further object of the present invention, there is alsoprovided a method of hardening the surface of pure titanium andtitanium-base alloys which comprises the step, in addition to the abovesteps, of aging the surface.

According to one aspect of the present invention, the foregoing andother objects are attained by a process for preparing surface-hardenedpure titanium and titanium-base alloys which comprises coating with asubstitutional metal having a larger diffusion constant than titanium,which can be more readily nitrided than titanium, which has a higherdensity than the base metal, which is rich in the stable beta phase, andwhich has a Vickers hardness of above 400.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of theinvention will be readily obtained as the same becomes better understoodby reference to the following detailed description when taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a microscopic photograph of the surface layer obtained bynitriding a titanium-base alloy (5% Al 2% Cr 1% Fe balance Ti) at atemperature of 850C. for hours in a nitrogen atmosphere;

FIGS. 2A and 2B are microscopic photographs of the surface layer of ironplates in thickness of 5 and 13 microns, respectively, on the surface ofthe titanium-base alloy and nitrided at a temperature of 850C. for 50hours in a nitrogen atmosphere;

FIG. 3 is a graph showing the relationship between the thickness of theiron plated layer and that of the hardened layer of titanium;

FIG. 4 is a microscopic photograph of the surface layer on thetitanium-base alloy shown in FIG. 28, age treated at a temperature of500C. for 6 hours in a vacuum atmosphere;

FIG. 5 is a graphical representation of the relationship between theVickers hardness and the distance from the surface of iron platedtitanium-base alloys shown in FIGS. 28 and 4;

FIG. 6 is a microscopic photograph of the surface of the aforementionedtitanium-base alloy, plated with a nickel layer having a thickness ofmicrons, and nitrided at a temperature of 850C. for 50 hours in anitrogen atmosphere;

FIG. 7 is a graph showing the relationship between the Vickers hardnessand the distance from the surface of the nickel or copper platedtitanium-base alloys shown in FIGS. 6 and 8; and,

FIG. 8 is a microscopic photograph of the surface of the aforesaidtitanium-base alloy, copper plated to a thickness of 10 microns, andnitrided at a temperature of 850C. for 50 hours in a nitrogenatmosphere.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In this method, metaldiffusion and nitriding are accomplished simultaneously. It follows thatthe substitutional metal is first coated onto the base, and then it isheated in an atmosphere of pure nitrogen gas or in a nitrogen-containingatmosphere, such as in a gas derived by cracking ammonia.

The pure titanium may be commercially pure metal, and the titanium-basealloy may be any of the Ti-Al-V types such as Ti-6Al-4V: Ti-Al-Sn typessuch as Ti-5Al- 2.5Sn; Ti-Al-Cr-Fe types such as Ti-5Al-2Cr-1Fe; Ti-Al-Mo-V types such as Ti-4Al- 3Mo-1V; Ti-Al-V-Sn types such asTi-6Al-6V-2Sn; Ti-l3V-l lCr-3Al, Til2Mo-6Zr-4.5Sn,Ti-2.25Al-llSn-5Zr-Mo-0.1Si, and other commercial titanium alloys.

The substitutional metal may be, for example, iron, nickel, copper,aluminum, manganese, chromium, molybdenum, silicon, or cobalt. The mostpreferred metal is one which can prevent the hardness at hightemperature from decreasing due to diffusion into the titanium, andwhich'can thicken the hardened layer. For this purpose, the ideal metalis one which has as large a diffusion constant as possible, relative totitanium, a large strengthening capacity, and is easily nitridable. Inconsidering the diffusion constant, strengthening capacity and nitridingcapacity, iron is the most preferred substitutional metal.

The coating can be applied by any convenient method such as plating,deposition, welding, discharge coating and painting. It is only requiredthat the coating be applied without undue concern over other existingproblems such as separation of the coated layer.

The coated titanium or titanium-base alloy is then heated in a purenitrogen atmosphere. Then, the coated metal is nitrided. It is thenheated at a raised temperature with the result that the coated metalstarts to diffuse into the titanium or titanium-base alloy in thenitrogen atmosphere. Thus, in addition to the coated metal on thetitanium or titanium-base alloy, nitrogen is also diffused into thetitanium or titanium-base alloy so that the hardened layer becomes aternary alloy metal of titanium-coated metal-nitrogen. Thus, thehardness of the surface layer becomes sufficiently high by thenitriding, and yet, since the coated metal is diffused into the base,the thickness of the surfacehardened layer is greater, aboveapproximately 150 microns, than the layer hardened only by nitrogen.Since the substitutional metal is diffused, its strengthening capacityis not reduced even at relatively high temperatures.

The nitriding conditions of the metal in a nitrogen atmosphere will varydepending upon the type of alloy used, but temperatures of 750l050C. forperiods of 10-100 hours are preferable. The particular conditions willdepend upon the particular shape and size of the treating material, andaccordingly may not always be within these stated ranges.

If the coated metal is a beta stabilizing element, it will diffuse toform an alloy with the titanium or titanium alloy of the base metal, andthe beta phase portion will increase with the result that it sometimesbecomes a beta single phase. However, if such a metastable beta phase isaged at temperatures of about 400600C., a fine alpha phase will beprecipitated to increase the hardness. Thus, the hardness of thesurface-hardened layer may be further increased while simultaneouslyenlarging the effective thickness of the hardened layer.

Having now generally described the invention, a further understandingcan be obtained by reference to certain specific Examples which areprovided herein for purposes of illustration only and are not intendedto be limiting unless otherwise specified.

EXAMPLE I:

Titanium-base alloy, iron plated and nitrided It is known that incomparing pure titanium with titanium-base alloys, pure titanium isnitrided more readily and accordingly the nitrided layer is thicker.Therefore, this Example has used a titanium-base alloy, which is moredifficultly nitrided.

lron was electrically plated by conventional methods to a thickness of Sto l3 microns on the surface of a titanium-base alloy (5% Al 2% Cr 1% Febalance Ti). The thusly iron-plated titanium-base alloy was at onceplaced in a heating furnace of nitrogen atmosphere to be nitrided.

Nitriding conditions:

50 hours +50 mmHg Temperature: Heating period: Atmospheric pressure:

The thus-obtained surface-hardened titanium-base alloy is shown in FIGS.2A and 2B. In comparison, a surface-hardened titanium-base alloy whichwas not plated but heated for hours under conditions otherwise the sameas above, is shown in FIG. 1.

As clearly seen from the microscopic photographs shown in FIGS. 1 and 2,the thickness of the surfacehardened layer obtained by the conventionalmethod shown in FIG. 1 is approximately 20 microns, while that obtainedaccording to the method of this invention shown in FIGS. 2A and 2B wasapproximately 65 microns for iron plated to a thickness of 5 microns,and was approximately microns for iron plated to a thickness of 13microns. Thus, the effect of the method of this invention is clear.

The relationship between the thickness of the iron plated layer and thatof the surface-hardened layer is proportional, as shown in FIG. 3, sothat the thickncr the plated layer is, the greater becomes the thicknessof the hardened layer. Thus, the thickness of the surface-hardened layermay freely be selected by adjusting the thickness of the plated layer.However, in order to prevent the surface-hardened layer from beingremoved by polishing, it is considered necessary to have asurface-hardened plate in a thickness of at least approximately 100microns, and accordingly it is required to plate in a thickness of aboveapproximately 8 microns prior to nitriding.

Next, the titanium-base alloy was iron plated to a thickness of 13microns thereon and was then nitrided. Thereafter, it was aged in vacuoat a temperature of 500C. for 6 hours. The result of this treatment ofthe titanium-base alloy is shown in FIGS. 4 and 5. It is clearly seenfrom this that aging provides a fine structure in the hardened layer incomparison with that which is not aged. As shown in FIG. 5, comparingthe hardened layer and the hardness of the titanium-base alloy with thatwhich is not aged, as illustrated by curve A, the Vickers hardnessbecame above 500 even in the interior 500 microns from the surface, asseen by curve B. Thus, according to this treatment, the surface wassuperiorly hardened, and the effect of this invention may be furtherimproved by the aging treatment.

EXAMPLE 2 Titanium-base alloy, nickel plated.

Nickel was plated to a thickness of microns on the surface of atitanium-base alloy (5% Al-2%Cr-1%Fe -balance Ti), and the titanium-basealloy was then nitrided under the same conditions as in Example 1. Theresults of this treatment of the alloy are shown in FIGS. 6 and 7.

As seen in FIG. 6, the nitrided layer is approximately 25 microns at theoutermost portion of the surface of the hardened titanium-base alloy.Next to the nitrided layer there exists a nickel diffused layer in athickness of approximately 100 microns, which has succeeded to increasethe thickness of the hardened layer in the same manner as in Example 1.As to the relationship between the hardness and the thickness of thehardened layer of the titanium-base alloy, the thickness of the layerhardened above Vickers hardness of 500 reached approximately 200microns. Thus, sufficient hardness of the surface-hardened titanium-basealloy may be provided. Since nickel is an abrupt eutectoid alloy elementfor titanium, the same effect as achieved in Example 1 may be providedby relatively shorter aging treatment.

EXAMPLE 3 Titanium-base alloy, copper plated.

Copper was plated to a thickness of 10 microns on the surface ofatitanium-base alloy (5%Al 2%Cr 1% Fe -balance Ti), and the titanium-basealloy was then nitrided under the same conditions as in Example 1. Theresults of the treatment of the alloy are shown in FIGS. 7 and 8.

The nitrided layer is approximately 25 microns at the outermost portionof the surface of the hardened titanium-base alloy, and next to thenitrided layer there exists a copper diffused layer having a thicknessof approximately 75 microns. Thus, a hardened layer which is thickerthan that obtained by conventional methods is obtained. As seen in FIG.7, the hardened layer on the surface of the titanium-base alloy isalmost the same as that in the case of 'nickel, but the thickness of thesurface-hardened layer is slightly thinner, such as approximately 100microns, and therefore it is considered to be necessary to form a copperplated layer thicker than about 10 microns. Aging treatment may providethe same effect as in Example 2.

It should be understood from the foregoing description that since themethod of the present invention simultaneously diffuses and nitrides thesubstitutional type metal entered into the titanium, the thickness ofthe surface-hardened layer becomes above 100 microns and is harder thanheretofore attainable, with the result that it is no longer a severedrawback when portions of the hardened surface are to be removed bypolishing. Yet, since the method provides a layer diffused with asubstitutional metal, it will not suffer a weakened strengtheningcapacity at high temperatures, with the result that problems ofseparation of the'plated layer from the surface-hardened layer, such asare encountered with the conventional plating method, do not exist.

It should also be understood that the thickness of the surface-hardenedlayer of the titanium-base alloy may be freely adjusted in response todesired requirements.

It will be appreciated that, while the foregoing disclosure relates onlyto preferred embodiments of the invention for preparing surface-hardenedtitanium or titanium-base alloys, numerous modifications or alterationswill be apparent to those skilled in the art without departing from thespirit and scope of the invention as set forth in the appended claims.

What is claimed as new and desired to be secured by letters patent ofthe United States is:

l. A surface hardened pure titanium or titanium base alloy, which isformed by coating a substitutional metal selected from the groupconsisting of Fe, Ni, and Cu, onto a titanium or titanium base alloymetal in sufticient thickness to provide a surface hardened layer of atleast 100 microns and heat treating said metal in a nitrogen atmosphereat a temperature of 750 1,050C. for periods of 10 100 hours whereby thecoated metal is nitrided and is diffused into the base metal andsimultaneously nitrogen is diffused into the base metal, whereby ahardened layer which is richer in stable beta phase than the base metaland which has a Vickers hardness of about 400 is obtained.

2. A surface-hardened titanium-base alloy as set forth in claim 1,wherein said alloy is of the Ti-Al-V type Ti- Al-Sn type, Ti-Al-Cr-Fetype, Ti-Al-Mo-V type, Ti-Al- V-Sn type, Ti-V-Cr-Al type, Ti-Mo-Zr-Sntype, or the Ti-Al-Sn-Zr-Mo-Si type.

3. A method of hardening the surface of pure titanium or a titanium basealloy metal, which comprises coating onto said base metal asubstitutional metal, selected from the group consisting of Fe, Ni, Cu,in sufficient thickness to provide a surface hardened layer of at least100 microns heating said coated metal in a nitrogen atmosphere at atemperature of 750 1,050C. for a period of 10 .100 hours, whereby thecoated metal is nitrided and is diffused into the base metal andsimultaneously nitrogen is diffused into the base metal whereby ahardened layer which is richer in stable beta phase than the base metaland which has a Vickers hardness of above 400 is obtained. 7

4. A method as set forth in claim 3, wherein said coating step isplating, depositing, welding, discharge coating, or painting.

5. A method as set forth in claim 3, further comprising the step ofaging.

6. A method as set forth in claim 3, wherein said aging step isconducted at a temperature of 400 C.

2. A surface-hardened titanium-base alloy as set forth in claim 1,wherein said alloy is of the Ti-Al-V type Ti-Al-Sn type, Ti-Al-Cr-Fetype, Ti-Al-Mo-V type, Ti-Al-V-Sn type, Ti-V-Cr-Al type, Ti-Mo-Zr-Sntype, or the Ti-Al-Sn-Zr-Mo-Si type.
 3. A method of hardening thesurface of pure titanium or a titanium base alloy metal, which comprisescoating onto said base metal a substitutional metal, selected from thegroup consisting of Fe, Ni, Cu, in sufficient thickness to provide asurface hardened layer of at least 100 microns heating said coated metalin a nitrogen atmosphere at a temperature of 750* - 1,050*C. for aperiod of 10 - 100 hours, whereby the coated metal is nitrided and isdiffused into the base metal and simultaneously nitrogen is diffusedinto the base metal whereby a hardened layer which is richer in stablebeta phase than the base metal and which has a Vicker''s hardness ofabove 400 is obtained.
 4. A method as set forth in claim 3, wherein saidcoating step is plating, depositing, welding, discharge coating, orpainting.
 5. A method as set forth in claim 3, further comprising thestep of aging.
 6. A method as set forth in claim 3, wherein said agingstep is conducted at a temperature of 400* - 60*C.